A brazing method is one kind of joining method for metal, ceramics etc., which falls into the category of welding method. In particular, the brazing method is a method of brazing using a brazing material having a melting point of not less than 450° C. and lower than the melting point of a base material to be brazed. Specifically, brazing is conducted in the following manner. A brazing material applied onto the surface of one base material is heated to a temperature at which the brazing material is melted but the base material is not melted, whereby the melted brazing material is attracted by the capillary phenomenon toward a joining surface of the other base material, to form a thin film of the brazing material. Thereafter, the brazing material is solidified by cooling to join the one base material to the other material.
Metal used generally as a brazing material in brazing is used in the form of a plate, foil, wire, powder, or powder-containing paste.
The brazing material in the paste form among these forms is used frequently because it is suitable for application to a site onto which the brazing material in the form of a plate, foil, wire or powder is hard to apply, that is, a site where the brazing material easily drops before or during brazing. The paste composition is composed essentially of a brazing powder, a binder and an organic solvent. As a binder solution, brazing cement or the like is commercially available. When the binder solution is used, it can be mixed with a commercial powdery brazing material to give a paste composition.
Brazing is carried out by using an industrial brazing furnace such as a reducing gas atmosphere furnace, an inert gas atmosphere furnace or a vacuum furnace. When a conventional paste composition is used in brazing, a carbon component remains on a brazed portion after brazing, depending on the type of brazing furnace, which may generate residual carbon etc. to cause brazing insufficiency. For example, a certain paste composition does not cause brazing insufficiency upon brazing in a furnace of a reducing gas atmosphere such as a hydrogen-containing gas, but causes significant brazing insufficiency such as residual carbon sometimes upon brazing in a furnace of an inert gas atmosphere such as argon gas, helium gas etc. or in a vacuum furnace. Accordingly, even if a paste composition suitable for a base material as a subject to be brazed is selected as a brazing material, the type of brazing furnace used is sometimes limited in order to prevent brazing insufficiency and to achieve an excellent outward appearance of a brazed portion. This is extremely inconvenient in industrial production.
At present, a paste composition for brazing which can, regardless of the type of industrial furnace, be applied to any type of brazing furnace has never been obtained.
Accordingly, an object of the present invention is to provide a paste composition for brazing, which upon brazing, does not cause brazing insufficiency such as residual carbon in any type of industrial brazing furnace, can achieve an excellent outward appearance of a brazed portion after brazing and can be applied to any existing industrial brazing furnaces, as well as a brazing method using the same.